Thermal printers are often used to print photographic quality images for both in-store and in-home applications. Thermal dye printers transfer dye onto a page by heating dye on a donor material and having it migrate onto a receiver material. Resistive print heads are one of the most common methods of heating the dye on the donor. Resistive print heads utilize a stripe of tiny resistors, which are heated by selectively sending current through each. The stripe of resistors image an entire row of the image at a time. By varying the power through the resistor, the amount of dye is modulated to transfer from no dye transfer (Dmin) to maximum transfer (Dmax) in a continuous fashion. Because of this, thermal printers are continuous tone printers and can make excellent photographic quality prints.
Thermal dye sublimation printers transfer the dye by heating the dye sufficiently such that it changes to a vapor state and migrates across a small air gap onto the receiver. Thermal dye diffusion printers heat the dye, changing it to a liquid state, and then the dye transfers to the receiver when the receiver comes into contact with the donor.
The heating and cooling of the individual resistors is not instantaneous. The delay in heating a resistor causes a feature edge printing during an increase in heating, also referred to herein as a “rising edge”, to appear blurry. The delay in cooling a resistor similarly causes a feature edge printed during a decrease in heating, also referred to herein as a “falling edge”, to appear blurry.
The problem of smear is not limited to thermal printing. Smear also affects printing methods that move paper over a plate bearing fluid ink and other imaging techniques. In offset printing applications, master cylinders or plates wear over time. That wear, in conjunction with high speed printing causes smear down the printed page comparable to thermal smear.
It is highly desirable to make printers print faster. This tends to increase smear. As thermal printers print faster, the time it takes to print a single row of an image, known as “line time”, decreases. As the line time decreases and the time to heat and cool a resistor stays relatively constant, the blurred rising and falling edges, which taking identical measures in the time domain, take up larger measures in distance, causing the blurred edges to stretch further down the printed page. The length of the blur is proportional to the decrease in line time. Blurrier edges have less low, middle, and high frequency content, and require increased sharpening to maintain sharpness in the resulting image. Unfortunately, the more an image is sharpened, the more any noise and artifacts in the image are also amplified.
Traditional thermal smear compensation techniques adjust the manner of heating and cooling the print elements of the thermal print head, thus compensating or partially compensating for smear as a part of the transferring of donor material. Innui et al, U.S. Pat. No. 4,574,293 discloses tracking heater pulses and adjusting pulse widths accordingly. Kawakami et al., U.S. Pat. No. 4,688,051 adjusts the pulse count. Saquib et al, U.S. Pat. No. 6,819,347 B2 discloses using a model of a thermal print head response to predict the temperature of each thermal print head element at the beginning of each print head cycle based on the ambient temperature of the print head, the thermal history of the print head, and the energy history. The amount of energy applied to each print element is then adjusted based on the predicted temperature at the beginning of the print head cycle. These approaches have the shortcoming that the compensation is the same throughout the image.
A wide variety of known algorithms, such as sharpening algorithms, can affect the higher spatial frequency image content. Joyce, U.S. Pat. No. 4,941,190 describes a method of modifying the gain of the center pixel of a sliding window as a function of the mean of the center pixel and its neighbors. Kwon et al., U.S. Pat. No. 5,081,692 discloses use of an unsharp masking method using center weighted local variance for image sharpening and noise suppression. The amplification factor of the difference between an original image and its blurred counterpart is changed using a pseudo center weighted variance. Ghaderi, U.S. Pat. No. 5,481,628 discloses using of a threshold value that is updated on a line-by-line basis and selecting between high-pass and low-pass filters dependant upon the local image, instead of using one filter type exclusively. Keyes et al., U.S. Pat. No. 6,091,861 discloses choosing the sharpening level based on the exposure, film type, grain estimate, and magnification of the original image. Keyes et al., U.S. Pat. No. 6,118,906, discloses using noise estimates of an original image to select sharpening levels. Wang et al., U.S. Pat. No. 6,424,730 discloses edge enhancement at specified high frequencies based on the human visual system. Shadow regions are enhanced more than highlight regions. Fiete, U.S. Published Patent Application 2005/0135697 A1 discloses a set of adjustable filters, in which each filter operates over a given spatial frequency region. These techniques can modify middle and high frequency content, but are not particularly effective in reducing smear. For example, sharpening algorithms are generally either circular or elliptically symmetric and sharpen uniformly across and down the page, and do not distinguish between rising and falling edges.
It would thus be desirable to provide printers and methods having digital filtering that do not have the above shortcomings.